Voltage is a measure of the amount of force that electricity can produce. The top speed of any electric motor is determined by the voltage (force), due to a technical thing called back-EMF (we won't bore you with the details here). Anyway, early electric bikes started at 12 or 24 Volts. But it turns out that most motors won't turn very fast at those low voltages. So, now, most electric bike motors run at 36 volts or higher. The voltage rating of a battery will relate to the overall amount of power a battery can output. So, a 48V battery can output more power than a 36V battery, so your top speed will be higher, you will get greater acceleration, and you will be able to power up hills at a higher speed.
Voltages above 50V may pose health hazards, so we urge caution when using any battery over 36V